As a conventional method for focus detection of cameras, the so-called phase-difference detection method is known. In the phase-difference detection method, images of light rays coming from a subject and passing through different exit pupils positions of the image sensing lens of a camera are formed on a pair of line sensors. The shift amount between the relative positions of subject images formed on the pair of line sensors is obtained to detect the defocus amount of the subject image and an image sensing lens is driven on the basis of the detected defocus amount (for example see Japanese Patent Laid-Open No. 09-054242).
Another known conventional focusing method is the so-called contrast detection method (or the hill-climbing detection method). In the contrast detection method, an image of a part of light rays coming from a subject through the image sensing lens of a camera is formed on an area sensor, the sharpness of the formed subject image is calculated, and a focusing lens is driven so that the highest sharpness is achieved (for example see Japanese Patent Laid-Open No. 2001-136437).
On the other hand, the semiconductor device manufacturing technology has been improving every year, and along with the improvement, the number of pixels of a line sensor or an area sensor used for focus detection (hereinafter referred to as a “focus detection sensor”) has been increasing. Although the precision of focus detection increases with the increasing number of pixels, the time required for reading signals from a focus detection sensor also increases and so does the shutter release time lag. As a result, sometimes the shutter cannot be released at the best moment of the photographic subject. To address this problem, various methods have been proposed for speeding up focus detection without impairing the precision of focus detection (for example see Japanese Patent Laid-Open No. 63-148215 and No. 3550599).
According to Japanese Patent Laid-Open No. 63-148215, because not all image signals obtained from the pixels of a line sensor are required for focus detection in practice, A/D conversion, which is a time-consuming process, is not applied to the signals from pixels in the regions of a line sensor unnecessary for focus detection, thereby speeding up focus detection.
Japanese Patent No. 3550599 takes into consideration the fact that A/D conversion can be performed concurrently with processing performed in a micro processing unit (MPU). This is possible because A/D converters are peripheral circuits of an MPU, which performs processing such as correction processing required for focus detection by using image signals read from line sensors with an operation circuit such as an MPU. By causing the MPU to perform correction of a line sensor image, which is separate processing necessary for focus detection, while concurrently A/D conversion is being performed, the total time required for focus detection can be reduced.
However, in the method described in the Japanese Patent Laid-Open No. 63-148215, the position on line sensor where an image of a subject is formed varies depending on the amount of defocus and, in addition, the range of regions that must be read also varies depending on the characteristics of subjects. Accordingly, the number of pixels required for focus detection processing varies depending on situations. Although the method can reduce the time required for focus detection if the number of pixels required for the focus detection processing is small, it cannot reduce the time if a large number of pixels are required for the focus detection processing. Furthermore, the usability is impaired by the release time lag that varies depending on subjects due to variations in the time required for reading pixels.
The method described in the Japanese Patent No. 3550599 cannot sufficiently reduce the time required for focus detection because processes that can be executed in parallel with A/D conversion are few.